Unfortunately, installation of an electrodeless lamp (e.g., an electrodeless fluorescent lamp) in a fixture with an electrically conductive outer shell results in significant variations in lamp performance, such as changes in lamp input power, output lumens, and ballast power loss. Changes in input power and output lumens are, to say the least, an inconvenience for the consumer, but changes in ballast power loss can significantly increase ballast temperature and shorten ballast life.
The changes in lamp performance upon installation in a fixture are caused by interaction between the electromagnetic fields produced by the excitation coil in the electrodeless fluorescent lamp and the conductive shell of the fixture. This interaction changes the impedance of the loaded drive coil as viewed from the ballast, and hence changes system performance.
A typical electrodeless fluorescent lamp ballast employs a resonant circuit. One approach to maintaining nominal performance of a resonant circuit is to use a feedback circuit in which an output variable is measured and fed back to a controller. In response to the feedback, the controller changes a control variable, such as input voltage or operating frequency, in such a manner that the circuit either runs with constant output power or operates at high efficiency. Disadvantageously, such feedback control schemes are too expensive to be practicable for the ballasts of electrodeless fluorescent lamps intended for use as incandescent lamp replacements.
Accordingly, it is desirable to provide apparatus for an electrodeless fluorescent lamp which allows the ballast to be adjusted for optimized performance outside of an electrically conductive fixture, while maintaining this performance even when the lamp is installed in a fixture that is electrically conductive.